 /*公共的样式 */

 * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
 }

 html, body {
     height: 100%;
     background-image: url(../image/background_image.png);
     background-repeat: no-repeat;
     background-position: center;
     background-size: cover;
 }

 .nav {
     height: 50px;  /* 导航栏高度 */
     background-color: rgb(50,50,50);
     color: white;
     line-height: 50px;   /* 行高  和导航栏高度一致时，文字就是居中的效果 */
     padding-left: 20px;  /* 左侧间隙 */
 }

 .container {
     width: 100%;
     height: calc(100% - 50px);

     display: flex;
     align-items: center;
     justify-content: center;
 }
